The present invention relates to a photopolymerizable elastomeric mixture for producing ozone-resistant flexographic printing forms, containing an elastomeric binder, an olefinically unsaturated compound polymerizable by free radicals, and a photoinitiator.
Elastomers, in particular those based on butadiene or isoprene, have been known for a long time for production of flexographic printing forms and are described, for example, in DE-B 2,215,090 (=U.S. application No. 4,423,135), DE-A 2,138,582 (=GB-A 1,358,062), 2,223,808 (=GB-A 1,395,822), DE-B 2,815,678 (=GB-A 1,577,706), DE-A 2,456,439 (=U.S. application No. 4,162,919) and 2,942,183 (=U.S. application No. 4,320,188). It was found, however, that flexographic printing forms containing these elastomers become brittle with even the slightest presence of ozone and show cracks.
Ozone, which represents a risk to these plates, is formed both during imagewise exposure of the photopolymerizable elastomeric layer, especially if UV light is used, and during use of the exposed and developed printing forms. In the latter case, particularly high ozone concentrations arise, especially when flexible packaging materials such as paper or plastics are to be printed. This is because, in order to ensure sufficiently strong adhesion of the printing ink, for example, to the plastic films, the latter are corona-treated before printing. Since considerable quantities of ozone are formed during the corona treatment and the treatment is most efficient when it takes place immediately before the printing step, the printing form is contacted with a considerable amount of ozone resulting from this process. The susceptibility of the developed printing form to ozone is further increased by the fact that a post-treatment of the printing forms with halogen, in particular with bromine, is carried out in most cases, in order to reduce their tackiness.
To improve the resistance of the printing form to ozone which forms cracks and promotes brittleness, various approaches have been followed.
According to DE-A 2,215,090, ozone resistance can be improved by additions to the photopolymerizable mixture. Microcrystalline wax and paraffin wax, dibutylthiourea, 1,1,3,3-tetra-methylthiourea, norbornene, N-phenyl-2-naphthylamine, unsaturated vegetable oils, ethylene/vinyl acetate copolymers, polyurethanes, chlorinated and/or chlorosulfonated polyethylenes, chlorinated ethylene/methacrylic acid copolymers, polypentadienes, furfural-derived resins, ethylene/propylidene rubbers, diethylene glycol esters of resins and copolymers of .alpha.-methylstyrene with vinyltoluene are proposed as additives.
The disadvantage of this method is that the additives are frequently inadequate to ensure the required resistance to ozone or, if the effective concentration of these additives is increased, the photopolymerizable mixture shows such cloudiness that satisfactory imaging is no longer feasible.
Another method for protecting a flexographic printing form from ozone is described in DE-A 3,512,632 (=U.S. application No. 4,680,25L). This method is not based on any additions to the photopolymerizable mixture, but describes a post-treatment of the printing form, which has been developed and treated with halogen, with certain polyglycol compounds. The polyglycol compounds described are either not etherified or only mono-etherified.
A major disadvantage of this method is the required additional treatment step. Also, the treatment reagents described leave much to be desired with respect to the achievable ozone resistance.